1. Field of the Invention
This invention relates to a membrane electrode unit with an integrated sealing edge for electrochemical cells which consists of a polymer electrolyte membrane and electrodes that cover it on both sides as well as thermoplastic sealing means deposited on the peripheral edge.
2. Description of the Prior Art
DE 197 03 214 describes a membrane electrode unit of the type described above, in which the sealing edge penetrates the electrodes in the peripheral area in question and adheres to the polymer electrolyte membrane. The sealing material used is a thermoplastic or thermosetting plastic which is formulated so that in the liquid phase it travels by capillary action through the pores into the electrodes to the polymer electrolyte membrane and adheres to it, thereby creating a secure seal. It has been found, however, that the ion-conducting polymer electrolyte membranes of the prior art which have ionic groups suspended on the polymer skeleton have a very low surface energy. Therefore they are very difficult to bond to other materials, which means that the adhesive and sealing action is limited.
The object of the invention is therefore to realize the sealing edge for a membrane electrode unit so that an effective sealing action is achieved on the polymer electrolyte membrane and at the openings to guide the media, and simultaneously to facilitate the handling of the membrane electrode unit during installation.
The invention teaches that this object can be accomplished with a membrane electrode unit of the type described in the introduction to claim 1, whereby the sealing edge consists of a layer of hot-melt-type adhesive that is deposited on both sides, and extends beyond the outer edge of the membrane electrode unit where it forms a one-piece projecting partial area, whereby the hot-melt-type adhesive contains ionic or strong polar groups for the generation of a surface interaction with the ionic groups of the polymer electrolyte membrane and thus a good adhesive action.
The invention thereby makes available a membrane electrode unit with a relatively stable sealing edge that adheres well to the membrane material and which, on account of its reinforcing action, also guarantees easy handling during installation and additionally makes possible a simple and effectively sealed realization of the openings for installation and to guide media In a fuel cell stack. Because the sealing edge is attached only to the outer peripheral area of the membrane electrode unit, the amount of membrane and electrode material required is relatively small. The good adherence and seal is achieved on one hand because the area that projects outward forms a one-piece unit, and on the other hand because the polar groups that are present at essentially regular intervals on the hydrocarbon skeleton of the hot-melt-type adhesive enter into a strong polar interaction with the membrane material which contains acid groups and thus produce a strong adhesive effect.
Additional features and advantageous developments and advantages of the invention are described in the dependent claims and in the following description of one embodiment of the invention which is presented purely by way of example.